Various cathodes have been studied for use in electrochemical reactions involving hydrogen-evolution. Since the technical breakthrough of corrosion-resistant valve metal electrodes, especially dimensionally stable anodes, many efforts have been made to obtain a valve metal supported bipolar electrode which could be activated over one surface with an anodically stable, electrocatalytic coating generally comprising a platinum group metal or platinum group metal oxide, and which could perform satisfactorily as a hydrogen evolution cathode over its other surface.
When hydrogen ions are cathodically discharged, hydrogen atoms are adsorbed on the surface and diffuse into the crystal lattice of the metal cathode, giving rise to the formation of hydrides which may precipitate at the grain boundaries within the metal structure.
Valve metal electrodes are badly affected by adsorbed hydrogen atoms which migrate into the valve metal and form hydrides, causing expansion of the valve metal lattice, weekening of its structure and peeling off of the electrocatalytic coating.
Proposals to solve this problem are described in U.S. Pat. No. 4,000,048, whereby the valve metal is coated with a layer of palladium-silver or palladium-lead alloy having a hydrogen desorption/adsorption ratio lower than unity. However, this involves the use of expensive noble metal cathodic coatings.
Recently, bipolar electrode assemblies with reportedly low hydrogen permeability rates have been proposed. U.S. Pat. No. 3,920,535 describes a multilayer composite comprising a valve metal plate coated with a suitable anodic material over one surface and with a silicon layer over the opposite surface, the silicon being protected by a metal coating suitable for the cathodic conditions. This silicon layer is intended to reduce hydrogen diffusion through the composite assembly, but it has a low electrical conductivity.
Another publication of interest is U.S. Pat. No. 3,884,792 relating also to multilayer metal electrodes having an intermediate layer of a metal substantially resistant to hydrogen diffusion. Generally speaking, the fabrication of known composite bipolar electrodes is complex and needs accurate control of the various coating processes to avoid damaging the adherence of previously applied layers.
U.S. Pat. No. 4,118,294 relates to a cathode composed of conductive powder embedded in a cured thermosetting resin, the cathodically operative surface being enriched with a hydrogen-evolution catalyst.
The various hydrogen-evolution cathodes and bipolar electrodes proposed hitherto nevertheless generally present several technical and economic limitations such as: high cost, complicated manufacture, unsatisfactory long-term electrocatalytic performance.